Many studies have been conducted on printing plates for the computer-to-palate system whose recent development is remarkable. Of these, as ones directed toward more process rationalization and the solution to the problem of waste liquid disposal, lithographic printing plate precursors that can be mounted on a printing machine and printed as such without development processing after exposure have been studied, and various methods have been proposed.
As one of methods dispensing with development processing, there is a method called on-machine (i.e., on-press) development in which an exposed printing plate precursor is mounted on a cylinder of a printing machine and a fountain solution and ink are supplied while rotating the cylinder, thereby removing a non-image area of the printing plate precursor. That is to say, this is a system in which the printing plate precursor is mounted as such after exposure and processing is completed in the ordinary printing process.
The printing plate precursor suitable for such on-machine development is required to have a light-sensitive layer soluble in a fountain solution and ink, and to have bright-room handling properties suitable for development on the printing machine placed in a bright room.
For example, Japanese Patent 2,938,387 discloses a lithographic printing plate precursor comprising a hydrophilic support having provided thereon a light-sensitive layer in which fine particles of a thermoplastic hydrophobic polymer are dispersed in a hydrophilic binder polymer. This patent describes that the fine particles of the thermoplastic hydrophobic polymer are exposed to an infrared laser beam to allow them to coalesce by heat in the lithographic printing plate precursor to form an image, followed by mounting of the precursor on a cylinder of a printing machine, thus being able to conduct on-machine development with a fountain solution and/or ink.
Further, Japanese Patent (Application) Laid-Open No. 127683/1997 and WO99-10186 also describe that thermoplastic fine particles are allowed to coalesce by heat, followed by the preparation of a printing plate by on-machine development.
However, the methods of forming images by fusion of the fine particles by heat as described above have the problems of low sensitivity and difficulty in obtaining high printing durability.